1. Field of the Invention
The present invention relates to a resistance welding machine and, more particularly, a resistance welding machine which is improved to avoid any damage caused by powders generated by a local wear of power feeding shaft which supports an arm.
2. Description of the Prior Art
Generally, in the resistance welder of arm gun type shown in FIG. 1, an arm 4 provided with an upper electrode 3 is rockably mounted through a shaft 5 on a main body 2 provided with a lower electrode 1. A pneumatic cylinder 7 is connected to the rear end of the arm 4 through a rod 6. The arrangement is such that the lower electrode 1 and the upper electrode 1 are moved toward and away from each other as the pneumatic cylinder 7 operates, thereby to clamp a work therebetween to effect the resistance welding.
The conventional means for supplying the upper electrode 1 with the electric power includes a flexible power feeding cable 8 connected to the arm 4. The flexible power feeding cable 8 is subjected to a repetitional bending force resulting in a shorter life.
In order to overcome this problem, it has been proposed to use the shaft 5 carrying the arm 4 as a power feeding shaft. More specifically, the power feeding cable is connected to the power feeding shaft which in turn is received by a bore of a power receiving bush fixed to the arm 4 so that the electric power is supplied to the arm 4 through a supporting structure for the latter including the power feeding shaft. In the known device of the type stated above, the whole part of the inner peripheral surface of the power receiving bush is finished at a high precision and the power feeding shaft is inserted into the bore of the bush such that it makes a sliding contact over the entire inner peripheral surface of the bush with a small clearance therebetween.
In consequence, when the arm 4 is rocked to press the upper electrode 3 against the work, a local wear is caused by a frictional engagement between the lower surface of the power feeding shaft and the inner peripheral surface of the bush. Unfortunately, due to the close fitting of the power feeding shaft to the bore of the bush, the powders generated as a result of the local wear cannot be released and are inconveniently accumulated in the small gap between the power feeding shaft and the bush. In consequence, is generated by the electric current which flows from the power feeding shaft to the power receiving bush to fuse and weld the powders to the power feeding section to damage the power feeding shaft and the power receiving bush during the rocking of the arm to increase the electric resistance against the current flowing from the power feeding shaft to the power receiving bush.